1. Field of the Invention
The invention relates to a print head capping device and, more particularly, to a technique for preventing breakage of a print head ink meniscus and for protecting the print head during transport.
2. Description of the Related Art
It is known to cap a print head with a capping device in order to prevent an ink jet nozzle of the print head from drying out. At the time of capping, if the temperature in the space enclosed by the print head and the capping device (hereinafter called the space in the cap) varies, the air in the space in the cap expands or contracts, thereby changing the pressure in the space in the cap.
At the end of the ink in the print head nozzle, a meniscus is formed that has a surface tension. The meniscus is liable to be broken at a very low pressure. If the pressure in the space in the cap increases with a temperature rise, the meniscus may be broken by the expanding air, which then, in the form of air bubbles, enters into the nozzle, and disturbs future ink ejection. Further, if the pressure in the space in the cap decreases with a drop in the temperature, the ink may be drawn into the cap through the nozzle, resulting in ink accumulation inside the cap. Consequently the nozzle surface will be smeared with the ink accumulated within the cap, so that when the ink is jetted, the direction of ink jetting deviates from the intended direction or the ink jetting itself is disturbed. Consequently, either one of a rise or drop in the temperature can result in defective printing.
Therefore, it is necessary to hold the pressure in the space in the cap to a low pressure at which the meniscus will not be broken. As techniques for so holding the pressure in the space in the cap, there are, as has been disclosed in the U.S. Pat. No. 4,684,963, (1) a capping device provided with a damper chamber connected to the cap section to absorb a pressure change in the space in the cap, and (2) a capping device in which the space in the cap is communicated with the outer atmosphere in order to prevent the occurrence of a pressure change.
During transport as well as shipping of a print head or a head unit from the point of manufacture, a preservation liquid having the same properties as the ink, exclusive of dyes and pigments, is placed in a print head, i.e., in each ink chamber formed in an actuator, and a manifold connected with each ink chamber for the purpose of smooth initial ink introduction at the time of the beginning of use. Further, to prevent leakage of the preservation liquid and to protect the nozzle from damage, the nozzle plate is capped with a protecting rubber or the like to keep the ink chamber and manifold interior hermetically sealed.
The above-described capping device (1), however, has drawback that other parts, such as a damper chamber, than the capping device must be attached to the capping device, making the device complicated in structure and accordingly large in size. In the capping device (2), since the space in the cap communicates with the atmosphere, the nozzle is likely to dry out. Also, in a conventional damper chamber, only walls constituting the damper chamber are partly made of a thin film and displaced to absorb a pressure change in the space in the cap, and therefore the space in the cap displaces little, resulting in a relatively low pressure holding effect for the capacity occupied by the damper chamber.
When the nozzle plate of the print head is capped with the protecting rubber or the like at the time of shipping, a pressure difference between the interior of the print head and the outside increases with an external pressure change and an internal pressure rise caused by the evaporation of the preservation liquid in the ink chambers and the manifold. As a result, the print head interior will communicate with the outer atmosphere through the lip section of the protecting rubber or the like, resulting in a dry nozzle.